不是所有的选择器都需要去分词，生成相应的匹配函数，这样流程比较复杂，当浏览器具备原生的方法去匹配元素是，没有理由不优先匹配，下面看看进入Sizzle后，它是怎么优先匹配这些元素的：

 

function Sizzle( selector, context, results, seed ) {    /*    执行$("ul.list>li span:eq(1)")时，递归第二次时，    selector    "ul.list>li span"    */    var match, elem, m, nodeType,        // QSA vars        i, groups, old, nid, newContext, newSelector;    if ( ( context ? context.ownerDocument || context : preferredDoc ) !== document ) {        setDocument( context );    }    context = context || document;    results = results || [];    if ( !selector || typeof selector !== "string" ) {        return results;    }    if ( (nodeType = context.nodeType) !== 1 && nodeType !== 9 ) {        return [];    }    if ( documentIsHTML && !seed ) {        // Shortcuts        if ( (match = rquickExpr.exec( selector )) ) {
    //先看是不是简单的ID选择器 TAG选择器 类选择器            // Speed-up: Sizzle("#ID")            if ( (m = match[1]) ) {                if ( nodeType === 9 ) {                    elem = context.getElementById( m );                    // Check parentNode to catch when Blackberry 4.6 returns                    // nodes that are no longer in the document #6963                    if ( elem && elem.parentNode ) {                        // Handle the case where IE, Opera, and Webkit return items                        // by name instead of ID                        if ( elem.id === m ) {                            results.push( elem );                            return results;                        }                    } else {                        return results;                    }                } else {                    // Context is not a document                    if ( context.ownerDocument && (elem = context.ownerDocument.getElementById( m )) &&                        contains( context, elem ) && elem.id === m ) {                        results.push( elem );                        return results;                    }                }            // Speed-up: Sizzle("TAG")            } else if ( match[2] ) {                push.apply( results, context.getElementsByTagName( selector ) );                return results;            // Speed-up: Sizzle(".CLASS")            } else if ( (m = match[3]) && support.getElementsByClassName && context.getElementsByClassName ) {                push.apply( results, context.getElementsByClassName( m ) );                return results;            }        }        // QSA path 是否支持document.querySelectorAll        if ( support.qsa && (!rbuggyQSA || !rbuggyQSA.test( selector )) ) {            nid = old = expando;            newContext = context;            newSelector = nodeType === 9 && selector;            // qSA works strangely on Element-rooted queries            // We can work around this by specifying an extra ID on the root            // and working up from there (Thanks to Andrew Dupont for the technique)            // IE 8 doesn't work on object elements            if ( nodeType === 1 && context.nodeName.toLowerCase() !== "object" ) {                groups = tokenize( selector );                if ( (old = context.getAttribute("id")) ) {                    nid = old.replace( rescape, "\\$&" );                } else {                    context.setAttribute( "id", nid );                }                nid = "[id='" + nid + "'] ";                i = groups.length;                while ( i-- ) {                    groups[i] = nid + toSelector( groups[i] );                }                newContext = rsibling.test( selector ) && context.parentNode || context;                newSelector = groups.join(",");            }            if ( newSelector ) {                try {                    push.apply( results,                        newContext.querySelectorAll( newSelector )                    );                    return results;                } catch(qsaError) {                } finally {                    if ( !old ) {                        context.removeAttribute("id");                    }                }            }        }    }    // All others    //去除选择器首位空格，并进入select函数    return select( selector.replace( rtrim, "$1" ), context, results, seed );}

先看：rquickExpr

rquickExpr = /^(?:#([\w-]+)|(\w+)|\.([\w-]+))$/,

当执行：match = rquickExpr.exec( selector )，产生一个长度为4的数组：其中match[1]ID匹配项，match[2]为tag匹配项，match[3]为类匹配项 比如：当执行$("div")时， match=["div", undefined, "div", undefined] 接下来就判断具体符合哪种选择器了，然后用原生方法直接获得。 如果不是上述三种选择器，那么开始试探querySelectorAll：

// QSA path 是否支持document.querySelectorAll        if ( support.qsa && (!rbuggyQSA || !rbuggyQSA.test( selector )) ) {

然后就就开始try尝试：

if ( newSelector ) {                try {                    push.apply( results,                        newContext.querySelectorAll( newSelector )                    );                    return results;                } catch(qsaError) {                } finally {                    if ( !old ) {                        context.removeAttribute("id");                    }                }            }

如果成功就返回results